1. Field of the Invention
The present invention relates to an image processing apparatus for transforming a half-tone image into a bi-level image by using a dither matrix.
2. Description of the Prior Art
A half-tone image can be binarized with use of a proper dither matrix to form a bi-level image, and the gradation thereof can be expressed with use of an area gradation processing. It is known that a high quality bi-level image can be obtained by using a dither matrix which is similar to dot printing employed widely in the printing technology. However, a false texture is liable to disproportionately emphasized in a bi-level image produced by dither processing.
A checkered pattern is an example of such texture. For example, FIG. 8 shows examples of dots at various densities in dot printing. A checkered pattern may appear at a density of 50%. In case of the dither processing, white portions and black ones may form a checkered pattern if the resolution of dots is low and the input density level or an output area ratio is 50%, as shown in FIG. 9. A checkered pattern may attract unwanted viewer attention and thereby detracts from the half tone image.
On the other hand, if a dot is deformed to avoid a checkered pattern, the shape of each dot becomes quite different from a circle and therefore, the gradation at a portion having a high or low density level is spoiled remarkably. Various kinds of image processing methods have been proposed in order to remove false gradation textures. For example, in Japanese patent laid-open publications JP-A No. 58-44861, No. 58-153455 and No. 58-205376, a difference between the maximum value and the minimum one of the density level is calculated in a block which consists of a plurality of pixels, and a dither matrix to be used is selected among a plurality of dither matrices according to the difference while all outputs are made white if the average density of the block is smaller than a predetermined value. According to a method disclosed in Japanese patent laid-open publication JP-A No. 58-136173, a plurality of dither matrices of various patterns are prepared and a dither matrix which makes a ratio of the low frequency components smallest is selected among them. In a facsimile disclosed in Japanese patent laid-open publication JP-A No. 58-137362, a dither matrix suitable for giving a high resolution of image and another matrix suitable for shortening a transmission time are provided and an operater selects either one of them by changing the mode. Further, according to a method disclosed in Japanese patent laid-open publication JP-A No. 58-75372, a dither matrix of divergence type is adopted so that black dots are concentrated by overlapping them to eradicate white dots.